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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

392

FATTY SUBSTANCES  (GENERAL  METHODS)

temperature reaches a maximum (4) and remains there for a few minutes.
The Maumene number is t-^t.

To obtain constant and comparable results it is necessary to operate
always exactly as described and to use acid of the exact density ; this may
be controlled by using 2oc.c. of distilled water in place of the oil, the value
50 being then obtained by the test. The oil and acid used must be left
for some time (about 30 minutes) to attain the temperature of the surround-
ing air.

In the case of a drying oil, it is convenient to dilute the oil suitably
with olive oil of a known Maumene value, the result obtained being then
p.operly corrected.

Solid fats are used in the fused state, the acid being at the ordinary
temperature; allowance is then made in the calculation for the respective
specific heats.

The results obtained for drying and non-drying oils with Tortelli's apparatus
are about 8-10 higher than those given by Jean's apparatus.

The results vary with the method xised for their determination and with
the nature of the substances themselves. In general, however, drying oils,
fish oils and fish-liver oils give values above zoo, semi- and non-drying oils
and blubber oils, values less than 100 (usually 30-90), and animal fats low values
(30-35).

Old or rancid oils and those which have been exposed to the air or heated
give values different from the fresh oils.

22. Drying Properties of Oils

Certain fixed oils, when exposed to the action of the air, thicken and
gradually dry, forming transparent and elastic pellicles like rubber. Such
oils are described as drying oils. Oils which either remain fluid or thicken but
little, even after long exposure to the air, are non-drying and those which
thicken and dry, although incompletely and slowly, are termed semi-drying.

The drying properties of an oil depend on its power to absorb, with
greater or less rapidity, atmospheric oxygen, so that the drying properties
of an oil may be determined from the quantity of oxygen absorbed and
from the rapidity of the absorption.

There are several methods of determining the absorption, the most
common being those of Livache and Bishop.

1.  LIVACHE'S METHOD.   Precipitated lead is prepared by immersing a
sheet of zinc in 10% lead acetate solution acidified with nitric acid, and
washing the lead precipitate formed with water, alcohol and ether and dry-
ing it in a vacuum over sulphuric acid.   A clock-glass with about i gram
of the lead on it is weighed and about 0-5 gram of the oil allowed to fall
in drops on to it, care being taken that the different drops do not unite.
The glass is reweighed and then exposed to the air in a well-ventilated and
lighted place at a constant temperature.    It is weighed from time to time
Until no further increase occurs, the maximum increase representing the
oxygen absorbed.

2.  BISHOP'S METHOD.   Pure manganese resinate is prepared by treating
the commercial product with petroleum ether, filtering the solution, dis-